Urea-Formaldehyde (UF) resins are widely used as a binder for lignocellulosic material. These formaldehyde-based resins are inexpensive, colorless, and are able to cure fast to form a rigid polymer, thereby providing the finished product with excellent physical properties.
A serious disadvantage of UF resin-bonded wood products is that they slowly emit formaldehyde into the surrounding environment. Due to environmental, health, and regulatory issues related to formaldehyde emissions from wood products, there is a continuing need for alternative formaldehyde-free binders.
A number of formaldehyde-free compositions have been developed for use as a binder for making wood products.
U.S. Pat. No. 4,395,504 discloses the use of formaldehyde-free adhesive system prepared by a reaction of a cyclic urea with glyoxal, for the manufacture of particleboard. Such a system, however, showed a rather slow cure and required low pH for the cure.
U.S. Pat. No. 5,059,488 shows an advantage of glutaraldehyde over glyoxal, when used in a reaction with cyclic urea. The patent discloses the use of glutaraldehyde-ethylene urea resins for wood panel manufacture. It was shown that this resin cured faster than glyoxal-ethylene urea resin, and the cure can be performed at a relatively high pH. However, the glutaraldehyde-based resins are quite expensive.
U.S. Pat. No. 4,692,478 describes a formaldehyde-free binder for particleboard and plywood prepared of carbohydrate raw material. The process comprises hydrolysis of the carbohydrate by a mineral acid, and then neutralizing the resin by ammonia. Although the raw materials are cheap and renewable, the reaction has to be performed at a very low pH (about 0.5) and a high pressure.
U.S. Pat. No. 6,822,042 also discloses the use of a carbohydrate material (corn syrup) for preparing a non-expensive wood adhesive. Advantages of this binder include strong bonding, low cost, and renewable raw material. However, a disadvantage is using isocyanate as a cross-linker for this composition. Toxicity of isocyanates makes them non-desirable for the wood products manufacture.
U.S. Pat. No. 6,599,455 describes a formaldehyde-free binder for producing particleboard containing curable thermoplastic co-polymers and cross-linkers selected from epoxy, isocyanate, N-methylol and ethylene carbonate compounds. Such compositions provide good strength and water resistance when cured, however, they are quite expensive.
U.S. Pat. No. 6,348,530 describes a formaldehyde-free binder for producing shaped wood articles comprising a mixture of hydroxyalkylated polyamines and polycarboxylic acids. The binder preparation is rather complicated and the composition is quite expensive.
U.S. Pat. No. 6,730,718 describes an aqueous dispersion adhesive suitable for bonding porous substrates such as wood panels. This binder comprises a mixture of two polymers: one is a polyvinyl ester, and second is a polymeric protective colloid, such as polyvinyl alcohol. Derivatives of multifunctional aldehydes, such as glyoxal and glutaraldehyde, are used as cross-linkers. The binder solution preferably comprises at least one cure catalyst or accelerator, such as aluminum chloride. The binder was used as a cold-cure wood adhesive applied to both sides. The final product exhibited good strength and water resistance.
Despite these disclosures, there is a growing need for new formaldehyde-free aqueous compositions suitable for use as a binder for wood products, such as plywood or particleboard. It is desirable that such curable compositions contain relatively high amount of non-volatiles, and at the same time are stable, fast-curing and do not emit any toxic fumes during the cure and afterwards.